INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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Bearing capacity of a geotextile-reinforced foundation Capacité d'appui d'une fondation géotextile- armé

VITO A. GUIDO, Ph.D., P.E., Assistant Professor, Civil Engineering, Cooper Union School of Engineering, New York, NY, USA G. L. BIESIADECKI, Structural Engineer, Mueser, Rutledge, Johnston Et DeSimone, New York, NY, USA M. J. SULLIVAN, Geotechnical Engineer, Frank H. Lehr Associates, East Orange, New Jersey, USA

SYNOPSIS The use of geotextiles as a reinforcing element in soils has gained widespread use throughout the world. One application of geotextile reinforcement is below shallow foundations. Presented herein are the results of laboratory model tests used to study the bearing capacity of shallow foundations reinforced with geotextiles. The parameters studied were the effect on the bearing capacity of a souare footing of depth below the footing of the first layer of reinforcement, the vertical spacing of the layers, the number of layers, the width size of the sheet of geotextile reinforcement and the tensile strength of the geotextile. For the tests performed, the bearing capacity of the soil reinforced with geotextiles was increased by a factor greater than three.

INTRODUCTION TABLE I

The present concept of reinforced earth is due Properti es of the Sands Used i n Model Tests largely, in part, to the work of Henri Vidal Ser.xes Series (1966). Although the original work done by Vidal A Tests B Tests tTniformity Coeffi ci ent. C used galvanized metal strips as the reinforcing 2. 5 1. 9 material, geotextiles can be used effectively. u Ef f ect i ve Si ze, D^Q,ra Reinforced earth has been used successfully in 0. 6xl O-5 8. 6xl 0"5 Medi an Si ze, D^g, m retaining walls, and roadbed and embanknent stabili­ 1. 8xl 0"4 1. 5xl 0"4 zation. Its application to shallow foundation problems has been studied experimentally by Speci fi c Gravi ty 2.63 2.66 Binquet and Lee (1975), Akinmusuru and Akinbo- Mi ni mum Dry Uni t Wei ght , Y , . .kN/m^ 13. 45 13. 10 lade (1981), Biesiadecki (1983), and Sullivan d m i n (1984). This paper will investigate the experi­ Maxi mum Dry Uni t Wei ght , Ydjnax/ kN/ m^ 16. 45 15. 65 mental study carried out by the authors on the effect on the bearing capacity of a square foot­ Dry Unit Weight , kN/m3 14.80 14.26 ing on sand reinforced with a geotextile. d Rel ati ve Densi ty, Dr, % 50 50

Angle of Internal Friction, 0’,° 35 36 EXPERIMENTAL LABORATORY MODEL

The model tests were performed in a scruare stif­ fened plexiglass box 1.22m wide with a depth of In order to obtain the full benefits of a geo­ 0.92m. Two series of tests were performed for a textile behaving as reinforcement, sufficient total of 70 tests, 25 from Series A and 45 from deformation of the fabric is required to mobil­ Series B. Each series used a different uni­ ize the fabrics1 tensile stress. These deforma­ formly graded sand. In Table I are shown the tions can be obtained in a soil which is rela­ properties of the two sands used. A 0.31m tively loose. However, placement of the soil in square footing was used. A vertical load was the test model with relative densities less than applied to the footing through the use of a 50%, with repetitiveness from test to test, was hydraulic jack and hand pump. The load was ap­ very difficult. Therefore, a relative density plied in small increments and the resulting of 50% was used, which is in the medium compact footing displacement measured by two dial gages, density range. In addition, a soil at relative placed at opposite corners of the footing. The densities greater than 50% would not have exhib­ differential settlement recorded by these gages ited the increase in bearing capacity, due to was very small for all tests performed. The fabric reinforcement, as dramatically. geotextiles used as reinforcing material are shown in Table II. All model tests were per­ formed with square sheets of geotextiles. A TEST RESULTS parameter defined as the width ratio, b/B; where b is the width of the geotextile and B The effect of five parameters on the bearing ca­ is the width of the footing was required. The pacity of a square footing on a so il reinforced square sheets of geotextile were placed con­ w ith geotextiles was studied: centrically under the square footing. The geo­ metry of the model is shown in Fig. 1. 1. The depth below the footing of the firs t

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c , for the unreinforced sand was 82.7 kPa at a layer of reinforcement, u. This was expressed o as a dimensionless ratio u/B, where B is the settlement, s, of 0.015m, or s/BoE 0.05. The width of the footing. Series B tests had an ultimate bearing pressure, 2. The vertical spacing of the layer of rein­ q for the sand of 99.3 kPa at 0.019m of settle- forcement, Az. A dimensionless ratio of Az/B was used. ment, or s/B of 0.063. Binquet and Lee (1975) 3. The number of layers of reinforcement, N. introduced a term, bearing capacity ratio (BCR), 4. The width size, b, of the square sheet of for convenience in expressing and comparing test reinforcement, expressed in dimensionless form data: as b/B. BCR = q/qo (1) 5. The tensile strength of the geotextile used as reinforcement. where a is as defined above and q is the bear- -o ing pressure of the reinforced soil at a settle­ ment corresponding to the settlement at the t abl e II ultimate bearing pressure for the unreinforced soil. Typical load-settlement curves, for Geot ext i l es UseH in Model Tes;ts Series A tests, are shown in Fig. 2 for differ­

Tensi l e ent values of N. Fabri c Thi ckness Manuf act ur er St rengt h Tr ade St ruct ure xl O4 (m) (kN) Name BEARING PRESSURE/ q0 AND q

E. I. DuPont de Typar Nonwoven 3..8 0. 67 Nemour s & Co. 3401

Crown Zel l erbach Fi bretex Nonwoven 27..9 1.16 400 Mirafi Inc. 600X Woven 7,.6 1.33

Phi l l i ps Supac Nonwoven 22,.9 1. 47 Fi bers Corp. 8 NP St apl e

Burlington In­ Bi -t ech Woven 7..6 1.76 dustri al Fabri cs *5013

Hoechst Fi bers Trevira Nonwoven 53..3 2.16 Industri es Spunbond S1155

Fig. 2 Load-Settlement Curves for Series A Tests, u/B=0.5, Az/B=0.25 and b/B=2

Since the soil is at a relative density of 50% and can be classified as having a medium compact density, the load-settlement curve for the unre­ inforced soil (N=0) has a shape similar to that for a general shear failure mode. It does not exhibit an abrupt curvature into a vertical tan­ gent because the soil is not in the compact to very compact density range. The load-settlement curves for the reinforced soil(N>0) have the classical shape for a local-shesr (punching) failure mode; this was substantiated when u was less than 0.051m and the top layer of fabric visibly had the outline of the footing impressed into it. Since radial tensile stresses develop in the soil directly beneath the footing, dissi­ Fig. 1 Geometry of Model pating with depth, the fabric reinforcement placed directly beneath the footing is in tension preventing the general-shear failure mode to take place. In order to obtain maximum benefit from Except for those tests, where the effect of the a geotextile functioning as reinforcement, suf­ tensile strength of the geotextile was investi­ ficient deformation of the fabric is requird to gated, all model tests were performed using the mobilize its tensile stress. As is shown in Dupont Typar 3401. In no model test was a Fig. 2 for small settlements, s/B less than fabric ever torn or damaged in any way due to 0.017, the load-settlement curve of the unrein­ load application. Reference tests for both the forced soil indicates that it is stiffer than Series A and Series B tests were performed for the reinforced soil. At small deformations, the the case of no fabric reinforcement. For the full benefits of the presence of the fabric are Series A tests, the ultimate bearing pressure, not exhibited since the fabric has not deformed

1 7 7 8 5 /C /3 sufficiently, whereas, the presence of the Effect of the Number of Lavers of Reinforcement- fabric has changed the failure mode from one of In Fig. 4 is shown the variation of the BCRwith general-shear to one of local-shear as indica­ the number of layers, N. Fig. 4a is for Series ted above. For this reason, the unreinforced A tests with b/B=2, and Fig. 4b is for Series B soil is stiffer than the reinforced soil at tests with b/B=3. For both of these curves when small settlements. However, at settlements N=4 the depth of the bottom layer of reinforce­ where s/B is greater than 0.017, the load- ment is at 1.25B. As the number of layers in­ settlement curve for the unreinforced soil is crease from 0 (unreinforced) to 3 there is a to the left of those for the reinforced soil. steady increase in the BCR, however, as the The fabric has deformed sufficiently to mobilize number of layers increases above 3 there is its tensile stress thereby increasing the load­ little change in the BCR. This is true for both ing carrying capacity of the soil above that of Series A and Series B tests. These results are the unreinforced condition. similar to those observed by Binquet and Lee (1975). and Akinmusuru and Akinbolade (1981). Effect of the Depth of the Top Layer and the In addition, when N=3 the botton layer of rein­ Vertical Spacing of the Layers of Beinforcement- forcement is at a depth of z=B indicating that If the reinforcement is kept within the effec­ the placement of reinforcement below a depth B tive depth (z/B=l), with the depth of the bot­ yields no added improvement in the bearing ca­ tom layer of reinforcement at a value equal to pacity. The soils for Series A and Series B B, the vertical spacing of the layer is tied tests had qQ = 82.7 kPa and 99.3 kPa, respec­ directly to the number of layers and the depth tively. One would expect that the Series A of the top layer of reinforcement by tests would exhibit larger increased benefits in bearing capacity due to reinforcement than the Az _ 1-(u/B) Series B tests. A comparison of Figs. 4a and 4b B N-l (2) would indicate a sharper increase in the BCR In Fig. 3 are shown the results for Series B with N and a higher peak value of the BCR for tests, of varying the depth of the first layer the Series A tests than the Series B tests; even of reinforcement, and Az/B varying with N and though b/B of the Series A tests is smaller than u/B according to Eq. (2). For a given value of the b/B of the Series B tests. N as u/B increases (Az/B decreases) the BCR de­ creases to a value at which a further increase in u/B does not change the BCR. This would in­ dicate that beyond a critical depth, the effect of the location of the top layer of reinforce­ ment is offset by the decrease in vertical spacing of the layers for a constant value of N (the density of the number of layers within a given volume of soil is increasing). In addi­ tion, for a given value of u/B as N increases (Az/B decreases) the BCR increases. Therefore, the effect of the two parameters u/B and Az/B on the bearing capacity cannot be considered separately, only concurrently.

Fig. 4 BCR Variation With Number Of Reinforcing Layers, u/B=0.5 and Az/B=0.25; (a) Series A Tests and b/B=2, (b) Series B Tests and b/B=3

Effect of the Width Size of the Square Sheet of Reinforcement - Since a zone exists beneath the footing in which radial tensile stresses dfevelbn» fabric placed within this zone will also be in tension. This fabric in tension reinforces the soil and increases the bearing capacity of the soil above the unreinforced condition (BCR increases). If fabric is placed outside this zone, it will be in compression and serve only as anchorage; it does not contribute to the re­ inforcing aspects of the soil. Therefore, to increase the width of the fabric beyond an opti­ mum value would yield no benefits of reinforce­ ment. The curve in Fig. 5 shows the variation in BCR with the width ratio b/B for Series A tests. As b/B varies from 0 (unreinforced) to Fig. 3 BCR Variation With Depth Of Top Layer 2.5 the BCR steadily increases, however, as the For Series B Tests, b/B=3 And Bottom b/B approaches 3 there is little change in the Layer Of Reinforcement at z=B BCR. This indicates that for given values of u/B, Az/B and N an optimum width ratio can be obtained to yield the optimum BCR.

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SUMMARY AND CONCLUSI ONS FABRI C : DU PONT TYPAR 3401 This paper has described two series of labora­ tory model tests to investigate the bearing capacity of a square footing on a geotextile reinforced sand. The results presented have shown that five parameters have a substantial effect on the bearing capacity; the depth below the footing of the first layer of reinforcement^ the vertical spacing of the layers, the number of layers, the width size of the sheet of geo­ textile, and its tensile strength. It was shown UI DTH RATI O, b / B that the parameters u/B, Az/B, and N are closely related if the reinforcement is kept within the Fig. 5 BCR Variation With Width Ratio For effective depth (z/B=l) with the depth of the Series A Tests, u/B=0.5, ¿z/B=0.25 bottom layer of reinforcement at a value equal and N=2 to B. In addition, similar trends in BCR values as a function of the above mentioned parameters for two different sands were obtained, indica­ ting that these trends are not specific for one Effect of Tensile Strength of Geotextile - sand only. This study has given a better under­ The reinforcements themselves must be able to standing of the geotextile reinforced earth slab, support tensile forces while the soil sustains where none previously existed. compressive and shearing stresses. Therefore, the tensile strength of the reinforcement is an important parameter, with increased BCR expected REFERENCES with increasing tensile strength. In Fig. 6 is shown the variation of BCR with tensile strength. Akinmusuru, J.O., and Akinbolade, J.A. (1981). As the tensile strength increases there is a Stability of Loaded Footings on Reinforced steady increase in the BCR, this is true for Soils. ASCE J. Geo. Engg. Div. (107), both the Series A and Series B tests. It was GT6, 16320, June, 819-827. indicated above that the unreinforced ultimate bearing pressure for the Series A tests was less Biesiadecki, G. L. (1983). A Study of the Bear­ than that for the Series B tests, therefore, it ing Capacity and Settlement Characteristics would be expected that the tensile strength of of Shallow Foundations Reinforced with Geo­ the fabric would have a greater effect on the textiles. Master's thesis presented to The BCR for the Series A tests than for the Series Cooper Union,School of Engineering, N.Y.,N.Y B tests. Fig. 6 indicates this. Some of the Binquet, J. and Lee, K.L. (1975). Bearing points in Fig. 6 are below the curves drawn, Capacity Tests on Reinforced Earth Slabs. indicating a bearing pressure that is too low. ASCE J. Geo. Engg. Div. (101), GT12, These points are for fabrics with large thick-- 11792, Dec., 1241-1255. nesses (as shown in Table II), therefore, the fabrics are more compressible and the settlement Sullivan, M. J. (19 84). Parameters Affecting the registered on the dial gages is not due solely Bearing Capacity and Settlement Character­ to the displacement of the soil but also the istics of Shallow Foundations Reinforced compression of the fabric. This would yield with Geotextiles. Master's thesis presented load-settlement curves where the bearing pres­ to the Cooper Union, School of Engineering, sures would be too low for a given settlement. N. Y., N.Y. Vidal, H. (1966) . La Terre Armee, Annales de l'Institut Technique de Batiment et des Traux Publics, July-Aug., 888-936.

Fig. 6 BCR Variation With Tensile Strength, u/B=0.5, Az/B=0.25, b/B=3, and N=3; (a) Series A Tests, (b) Series B Tests

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